The electric motor is gaining increasing importance as a replacement for or as a complement to internal combustion engines for driving motor vehicles. This is happening inter alia because of the environmental compatibility and the energy efficiency. The electric motor has still further advantages compared to the internal combustion engine, e.g. the full availability of the torque at low revolution rates and the lower noise generation.
The electrical energy that is required during operation of the motor vehicle is usually stored in batteries or accumulators. Said accumulators can e.g. be charged at charging stations provided especially for this purpose through a connection to the power supply system. A disadvantage with purely electrically powered vehicles is reduced range in comparison to motor vehicles that are powered by internal combustion engine. Mechanisms are usually therefore provided, with which the motor vehicle can be supplied with electrical energy or with which the batteries can be recharged even while in running operation. This is e.g. possible by means of the partial recovery of the kinetic energy during deceleration of the vehicle. The electric motor is operated as a generator during this. In this way the net traveling time, during which the motor vehicle can be operated purely electrically, increases. However, even in systems, e.g. hybrid systems, in which the electric motor is acting as a complement to an internal combustion engine, this type of energy regeneration can be usefully employed.
In addition to the recuperative energy generation and the resulting braking of the motor vehicle, conventional operating brakes are also provided for such vehicles, which have two separate associated brake circuits. Said operating brakes can e.g. be of hydraulic, electrohydraulic or electromechanical design. Their presence is on the one hand important in driving situations or operating situations in which the target braking torque intended by the driver exceeds the braking torque actually available from the generator. On the other hand, even if the generator fails or in situations in which the generator cannot supply any significant braking torque, safe and timely braking of the motor vehicle must be guaranteed.
Regenerative brake systems for recuperative regeneration of energy can be implemented in technically different ways. For example, such a brake system can be designed as an electrohydraulic brake system having two brake circuits and respectively associated hydraulic operating brakes, wherein the brake pedal is connected to an actuating unit and wherein a hydraulic control and regulating unit is provided, which conducts brake fluid into the operating brakes in the event of a braking demand by the driver.
Modern brake systems contain further extended functionalities, e.g. such as an anti-lock brake system (ABS) and/or an electronic stability program (ESP), by means of which pressure can be built up and reduced in a controlled manner in the individual brakes, whereby the vehicle can be stabilized during braking or if there is a danger of skidding.
In order to implement such functionalities, the brake system usually comprises at least one volume reservoir for brake fluid, which is normally in the form of a low pressure reservoir. Furthermore, valves are provided, by means of which brake fluid can be drawn from the brakes into the corresponding low pressure reservoir in a controlled manner, whereby the brake pressure is reduced. Hydraulic pumps are usually used for the active recovery of said volume of brake fluid.
In order to utilize the maximum available generator braking torque with such a brake system and hence also the maximum possible energy yield, the operation of the brake pedal must be decoupled from direct control of the wheel brakes. This relates to vehicles with a full or partial electric drive, which is designed for energy generation by means of regenerative brakes. Design solutions for this purpose are already known, whose disadvantage however consists in that mechanical free travel is provided in the transmission between brake pedal and wheel brakes, which cannot be adapted to the actual generator braking effect and in particular is to be taken into account as a missing volume in the event of a fault in the brake system. Depending on the situation, i.e. depending on the vehicle speed, the desired braking torque and the currently available generator braking torque, this means inter alia that for the driver of the vehicle a different pedal displacement results for the desired braking torque. The driver can be confused by this, because depending on the driving situation the same operation of the brake pedal leads to different braking behavior of the vehicle.